The real problem is most people later in life are full of friends and thus don't really need more. They have and used those social skills in school (including college), but then settled into life and made friends. As one friend moves away they will need a different one, but that doesn't happen often and so they don't need to make friends. It isn't that they are unfriendly, just that they already have a social life as full as they want it.
A social life is a dynamic, changing thing. "Full" vs. "empty" is an obvious way of measuring it, but I think "stagnant" vs. "dynamic" is a more important metric that we don't take into account as readily.
It's quite possible to have a "full" but stagnant social life (which is where I think some older software folks with families are finding themselves). It's also possible to have a dynamic social life without large numbers of people involved.
To avoid stagnancy, you always need new friends. This doesn't mean you have to kick others out. It just means accepting the natural flow of friends and relationships in and out of our lives, and accepting that this is something we don't have full control of.
